Sex is pivotal for reproduction, healthcare and evolution. In the fish medaka, the Y-chromosomal dmy (also dmrt1bY) serves the sex determiner, which activates dmrt1 for male sex maintenance. However, how dmy makes the male decision via initiating testicular differentiation has remained unknown. Here we report that autosomal gsdf serves a male sex initiator. Gene addition and deletion revealed that gsdf was necessary and sufficient for maleness via initiating testicular differentiation. We show that gsdf transcription is activated directly by dmy. These results establish the autosomal gsdf as the first male sex initiator. We propose that dmy determines maleness through activating gsdf and dmrt1 without its own participation in developmental processes of sex initiation and maintenance. gsdf may easily become a sex determiner or other autosomal genes can be recruited as new sex determiners to initiate gsdf expression. Our findings offer new insights into molecular mechanisms underlying sex development and evolution of sex-controlling genes in vertebrates.